1. Field of the Invention
The invention generally relates to an epitaxial structure of a gallium nitride semiconductor device and a process of manufacturing the same, and more particularly to a process of growing an epitaxial layer as a buffer layer.
2. Description of the Related Art
Illumination devices can be formed of different layers. In the case that the layers are epitaxially formed, defects are inevitably generated in their crystal structures. Performance of the illumination devices is adversely affected thereby in several aspects such as, for example, reduced illumination efficiency, lowered electron shifting, and prolonged diffusion paths for dopants. Moreover, V-shaped trenches, resulting in layer dislocation, appear in the quantum wells of active layers. Furthermore, initial reverse bias is also increased. If cracks or gaps appear in the crystal structure, an illumination device cannot be grown above the cracks or gaps, because the illumination device formed on such an area has a short service life and low illumination efficiency. Therefore, how to form an epitaxial layer with a perfect crystal structure is the key to improve the performance of the illumination device.
Gallium nitride series material can be used as a wide-bandgap semiconductor device that emits various lights, from green to violet. Gallium nitride bulks are difficult to grow so that, currently, gallium nitride must be formed on a sapphire- or SiC-based substrate. Lattice constant of the substrate is not consistent with that of gallium nitride. The gallium nitride layer directly formed on the substrate is not reliable and therefore a buffer layer must be formed between the substrate and the gallium nitride. The buffer layer is also called a nucleation layer with a lattice constant similar to that of the substrate. Nucleation and growth of gallium nitride are performed on the buffer layer to form a crystal structure significantly the same as the substrate, thereby increasing the crystallization of the gallium nitride-series layer. Therefore, the quality of the buffer layer greatly influences epitaxy of a cladding layer and an active layer subsequently formed, and indirectly influences properties of the illumination device.
U.S. Pat. No. 5,290,393 discloses a gallium nitride grown on a sapphire-based substrate, as shown in FIG. 1. A low-temperature aluminum nitride buffer layer 12 of 0.001–0.5 μm in thickness is formed on the sapphire-based substrate 11. A high-temperature aluminum gallium nitride buffer layer 13 is formed on the low-temperature aluminum nitride buffer layer 12. Generally, the temperature at which the low-temperature aluminum nitride buffer layer 12 is grown ranges from 200° C. to 900° C. The temperature at which the high-temperature aluminum gallium nitride buffer layer 13 is grown ranges from 900° C. to 1150° C. This method increases the crystallization of gallium nitride series compound. However, the defect density of 4 μm-thick aluminum gallium nitride layer 13 is still as high as 109–1010cm−2. U.S. Pat. No. 6,252,261 uses an ELOG method to reduce the defect density, as shown in FIG. 2. A base layer 22 of gallium nitride is epitaxially grown by MOCVD on a sapphire-based substrate 21. The base layer 22 includes a low-temperature gallium nitride buffer layer and a high-temperature gallium nitride epitaxial layer. The substrate is taken out from a MOCVD chamber. A (SiO2)23 mask having 1–120 stripes partially overlaps the base layer 22. Then, HVPE or MOCVD is performed to grow epitaxially a high-temperature gallium nitride epitaxial layer 24. With the user of the (SiO2)23 mask, the epitaxial growing mechanism is selectively performed. The growth direction of the epitaxial layer vertical to that of the gallium nitride in the areas of the base layer not covered by the (SiO2)23 mask. After the formed epitaxial layer reaches the same level as the mask, the layer is continuously grown, faster than before, in the growth direction of the gallium nitride base layer, thereby preventing defects from spreading in the vertical direction. The defect density of the subsequently epitaxially grown gallium nitride layer is therefore reduced. However, the defect density is not reduced until the layer has a thickness more than 10 μm. This kind of lateral epitaxial growth effectively reduces the defect density, but complicates the production of the mask. Meanwhile, the selective growth mechanism increases the whole production cost.
U.S. Pat. No. 6,475,882 discloses a lateral epitaxial growth using a SiN micro-mask. Before an epitaxial process is conducted, precursors including SiH4 and NH3 form SiN islands on a sapphire-based substrate. The SiN islands are used as a mask for subsequent lateral epitaxial growth to reduce the defect density. According to this disclosure, the flow and the reaction time of the precursors are controlled so as to obtain an epitaxial film with good crystallization. However, the uniformity and the density of the SiN micro-mask are not easily controlled, and the yield is not easily controlled, either.
Therefore, there is a need for a process of forming an epitaxial layer, suitable for the formation of an epitaxial layer of an illumination device, the epitaxial layer having perfect crystallization and little dislocation, low cost and improved yield.